This invention is directed generally to control of indoor climate modifying apparatus such as an air conditioning unit or a furnace for maintaining comfort for the occupants of enclosures. More specifically, the invention is directed to controlling operation of a climate control system for maintaining within desired limits the temperature and humidity in an enclosure. The discussion and disclosure following will be based primarily on the air conditioning case. However, one of ordinary skill in the art could easily adapt the invention for other systems. The invention will typically be implemented in an electronic thermostat which uses a microcontroller in conjunction with a temperature sensor for controlling opening and closing of a solid state switch which controls the flow of operating current to the air conditioning control module.
Thermostats typically in use now which direct operation of air conditioners use dry-bulb temperature as the controlled variable. The term "dry-bulb temperature" is defined as the actual temperature of the air as measured by a typical thermometer. The use of the term "temperature" or "air temperature" hereafter will refer to dry-bulb temperature unless the context clearly directs otherwise. It is easy to measure air temperature and this measurement is already available in most thermostats. A typical thermostat in air conditioning mode causes the air conditioning to begin operating when temperature rises above a set point value. The air conditioner responds by injecting cold air into the enclosure until the temperature within the enclosure has fallen to a point below the set point value. The typical thermostat uses an anticipation element so as to turn off the air conditioning before the actual set point is reached. For many situations this type of control results in air which is comfortable for the enclosure's occupants.
It is well known that an air conditioner removes humidity from the air as well as cools it. The mechanism by which humidity is removed involves passing air from the enclosure or from the outside through the air conditioner, reducing the temperature of this air to substantially less than the comfort range of 70.degree.-74.degree. F. In order to remove humidity from the air, the temperature of at least some of it must be lowered to less than the current dew point temperature, the temperature at which water condenses from the air. Some of the water in the conditioned air condenses on the cooling coils of the air conditioner in this process and drips off the coils to a pan below, from which it drains. Because air will not release any of its humidity until it has reached 100% relative humidity, i.e., its dew point temperature for condensation to occur, it is necessary for at least the air adjacent the cooled surfaces of the heat exchanger to reach this temperature. In normal operation the total air stream through the air conditioner may not reach 100% relative humidity because not all of the air is cooled to its dew point. The relatively cold and dry conditioned air (relatively dry even though it has nearly 100% relative humidity) is mixed with the uncomfortably warm and humid air within the enclosure to achieve a more acceptable 40-60% relative humidity at a comfortable temperature of 70.degree.-75.degree. F.
Normally this procedure results in air within the enclosure whose humidity is within the comfort range. However, there are situations that can result in air having humidity which is still too high when the temperature requirement has been met. To achieve air at comfortable levels of both temperature and humidity, an air conditioner is sized for the expected load which the enclosure will present so that when the set point temperature is reached, humidity is acceptable. But in cases of unusually high humidity or where the air conditioner capacity relative to the current environmental conditions does not result in sufficient dehumidification when the set point temperature is reached, it is possible for the air in the enclosure to have excessive humidity.
It seems to be a simple solution to control the relative humidity in the enclosure by simply adding a relative humidity sensor to the thermostat, and then controlling the air conditioner to hold relative humidity within a selected set point range. A problem with this approach is that the relative humidity of the enclosure air may actually rise as the air is cooled and dehumidified within the enclosure. This possibility arises because the relative humidity is a function of both the amount of water vapor in a given volume or mass of air and its dry-bulb temperature. Relative humidity for any volume of air is defined as the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that temperature. Since the vapor pressure of saturated steam drops rapidly with temperature, a relatively small amount of water vapor in a volume of air at a lower temperature can result in 100% relative humidity. It is thus possible to have a runaway situation where the humidity control function in the thermostat continues to call for further dehumidification, and as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on.
U.S. Pat. No. 3,651,864 (Maddox) teaches an air conditioning system which controls the relative humidity of enclosure air independently of the dry-bulb temperature. Maddox provides a humidistat responsive to relative humidity which operates in parallel with the normal dry-bulb temperature control. Because of the parallel operation of the two control functions, undesirable short cycles are possible. Furthermore, as previously mentioned, the relative humidity of the enclosure air may actually rise as the air is cooled and dehumidified within the enclosure. It is thus possible to have a runaway situation where the relative humidity control function as provided by the humidistat continues to call for further dehumidification, and as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on. These problems are solved by the present invention.
U.S. Pat. No. 5,345,776 (Komazaki et. al.) teaches a dehumidifying air conditioning system which utilizes two refrigerant heat exchangers supplied from the same compressor used sequentially on the conditioned air as a cooler/dehumidifier and reheater to control both relative humidity and dry-bulb temperature of enclosure air. A fuzzy logic controller is used to vary the compressor speed and the speed of the outdoor fan as a function of the measured relative humidity and dry-bulb temperature. As previously mentioned, the relative humidity of the enclosure air will actually rise as the air is cooled and dehumidified within the enclosure. It is thus possible to have a runaway situation where as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on. It is likely that in order to circumvent the mentioned runaway situation, it would be necessary to operate both indoor coils, viz., cooler/dehumidifier and reheater, simultaneously. The method described in U.S. Pat. No. 5,345,776 is more complicated by design when compared to commercially available conventional air conditioning units, including heat pump system, and requires more sophisticated controls and expensive hardware just for system operation. These problems are solved by the present invention which does not require any modifications to commercially available conventional air conditioning units, including heat pump system, and therefore can be easily and readily used in new and retrofit applications. Furthermore, the controls provided by the present invention is much simpler and will be substantially more robust in nature.
U.S. Pat. No. 4,105,063 (Bergt) is related art which discloses an air conditioning system which controls the dew-point temperature of enclosure air independently of the dry-bulb temperature. Bergt provides a sensor responsive to absolute moisture content which operates in parallel with the normal dry-bulb temperature control. Because of the parallel operation of the two control functions, undesirable short cycles are possible. This over-cycling problem is solved by the present invention.
U.S. Pat. No. 4,889,280 (Grald and MacArthur) is related art disclosing an auctioneering controller wherein the predetermined dry-bulb temperature set point is modified in response to a absolute humidity error signal. The enclosure temperature which results may not always be comfortable, and there is also a potential for overcycling.
U.S. Pat. No. 5,346,129 issued to this inventor and hereby incorporated by reference discloses a controller for a climate control system which has a relative humidity sensor as well as a dry-bulb temperature sensor within the enclosure. The relative humidity and dry-bulb temperature are used to determine a humidity (dew-point or wet-bulb) temperature. The humidity temperature value is used in conjunction with the dry-bulb temperature to generate a single error signal which is a function of both the dry-bulb and the humidity temperature values. This permits control of both enclosure temperature and enclosure humidity without abnormal cycling of the climate control system. The system as disclosed in U.S. Pat. No. 5,346,129 bases the error value on a function of the humidity temperature error and the dry-bulb temperature error. Experience has demonstrated that under certain circumstances the dry-bulb temperature within the enclosure can get reduced to a value significantly below the desired dry-bulb temperature set point as specified by the occupant in the enclosure. The inventor has further improved upon the '129 patent in U.S. patent application Ser. No. 08/664,012 now U.S. Pat. No. 5,737,934 filed Jun. 12, 1996 entitled, "Thermal Comfort Control" and in U.S. patent application Ser. No. 08/609,407 now U.S. Pat. No. 5,675,979 filed Mar. 1, 1999 entitled, "Enthalpy Based Thermal Comfort Controller". Both applications are currently copending, co-owned and hereby incorporated by reference. The present invention is an improvement upon these earlier invention by providing a reheat function only under certain operating conditions to overcome the reduced dry-bulb temperature.